Slackened needle thread assuring unit
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a slackened needle thread assuring unit capable of drawing out the needle thread necessary for a stitching operation. 
     2. Description of Prior Art 
     A known sewing machine is provided with a cutter for cutting a needle thread and a bobbin thread and with a thread end holder. 
     The cutter for cutting the needle thread and the bobbin thread permits a needle to be positioned at an upper stop position when the needle is raised from the surface of the sewn cloth after completion of the stitching operation and also permits a knife of the cutter to automatically cut off the needle and the bobbin threads at the position directly under the sewn cloth, namely, under a throat plate on which the sewn cloth is placed. The machine provided with the cutter is called a thread end cutting sewing machine. The thread end holder clamps the end portion of the needle thread which is cut off by the cutter to assure proper formation of a first stitch on the cloth to be sewn. 
     However, the prior art sewing machine is subject to a thread end breakage problem which can occur when the sewing operation is started from the state where by needle thread and the bobbin thread are cut off short. Then, the tension of the needle thread is abnormally increased and the end of the lower thread is drawn over the upper surface of the sewn cloth whereby the first stitch is not formed by a first needle hole, and in the worst case, the needle thread is cut off. This occurs because the turning angle of the upper shaft of the sewing machine increases to 70 degrees. Because of the structure of the sewing machine, the distance between a hole of the needle and a thread hole of the thread takeup is enlarged, causing a shortage in the slack of the needle thread when the needle is moved from the upper stop position to a lowering step at the start of the sewing operation. As a result, as manual finishing process is required after completion of the sewing operation. An end breakage can occur when an excessive tension is applied to the portion of the thread extended between the thread tension member and the needle hole via the thread takeup. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a slackened needle thread assuring unit which can prevent a bobbin thread from being drawn over a surface of a sewn cloth. 
     It is a further object of the present invention to provide a slackened needle thread assuring unit which can form a desired stitch. 
     It is still a further object of the present invention to provide a slackened needle thread assuring unit which improves the working efficiency of a sewing machine. 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRITPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a slackened needle thread assuring unit according to an embodiment of the present invention; 
     FIG. 2 is a perspective view showing a support plate, which is a constituent of the slackened needle thread assuring unit of FIG. 1; and 
     FIG. 3 is a very similar to FIG. 1 illustrating the operation of the slackened needle thread assuring units of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A slackened needle thread assuring unit according to a preferred embodiment of the present invention will be described with reference to FIG. 1 and 2. 
     A slackened needle and thread assuring unit comprises a frame 1 mounted on a bed of a sewing machine. The frame 1 has front, side and bottom portions or walls. A needle bar 2 has a needle 3 and is mounted on the bottom portion of the frame 1. The bar is vertically driven with interlocking with a main shaft. A thread tension member 5 is mounted on the side portion of the frame 1 for applying to a needle thread NT (described later) a predetermined tension and also for drawing and drawing the needle thread NT upward. A thread takeup 4 is positioned above said thread tension member 5. Thread takeup 4 has a predetermined relative position with respect to the needle bar 2 and is vertically movable in a specific manner for allowing the needle thread NT to slacken at an appropriate value necessary for normal stitching operation during normal sewing operation and also to draw or tighten the needle thread NT. A support plate 12 is attached to the front portion of the frame 1. Needle thread NT is fed from a spool holder base mounted on a table of the sewing machine. Thread NT is successively wound between thread tension disks of the thread tension member 5 and supported elastically by a thread tension spring 5a. The thread NT thereafter passes through a middle thread guide 8, the thread end hole 4a of the thread takeup 4, and a thread guide 9 on the lower end of the needle bar 2, and reaches the thread and hole 3a of the needle 3. A needle/bobbin thread cutter 7 is positioned apart from and under the frame 1 for cutting the needle thread NT. A slackened needle thread assuring unit 11 is mounted on the frame 1 and is composed of a thread end holer 10 positioned adjacent to the needle bar 2 for holding the end of the thread which is cut by the needle/bobbin thread cutter 7. A thread guide 22 extends across the front portion of the frame 1 with one end supported by the support plate 12 and the other free end defining a hook 22a. Hook 22a hooks the thread threaded into the thread tension member 5, the thread and takeup 4, and the needle 3. A drive means is supported by the support plate 12 for reciprocally moving the hook 22a in the direction substantially perpendicular to the threading direction of the needle thread NT. 
     The arrangement will be described more in detail. 
     The slackened needle thread assuring unit 11 is disposed on the front surface of the frame 1. That is, a support plate 12 is provided on the front surface of the frame 1 by a plurality of set screws 12a. The support 12 comprises, as shown in FIG. 2, securing portions 12b, 12b to be secured to the frame 1, a vertical surface portion 12c, a horizontal surface portion 12d, and an auxiliary horizontal surface 12e  positioned under the horizontal surface portions 12d. All of these portions are formed by bending a flat plate. The horizontal surface portion 12d has a hole 12f through which a solenoid 13 is secured thereto. The solenoid 13 has a cylinder rod (core) 14 extending downward under the horizontal surface portion 12d which is provided with a tip end rotatably connected to an end of a bell crank 17 by a pin. The bell crank 17 has holes 16 at the lower portion thereof through which a pin 19 is inserted. The pin 19 is rotatably supported by the vertical surface portion 12c of the support plate 12. A connecting link 21 has one end pivotally connected to an upper portion of the bell crank 17 by a pin 18 and the other end pivotally connected to one end of an operation arm 23 by a pin 20. The operation arm 23 has a middle portion pivotally connected to the auxiliary horizontal surface portion 12e of the support plate 12 by a hinge screw 25, one end secured to the auxiliary horizontal surface portion 12e, and the other end urged clockwise (in FIG. 1) by a spring 15 provided at the side of the other end of the operation arm 23. The operation arm 23 is always in an inoperative position when the cylinder rod 14 is under the horizontal surface portion 12d via the connecting link 21 and the bell crank 17. With such an arrangement, the drive means for reciprocally moving the thread guard 22 is structured. 
     The thread guide 22 of L-shape as a whole has a hook 22a formed by bending a tip end thereof in U-shape and has a base end detachably mounted on the other end of the operation arm 23. The hook 22a extends horizontally at the side portion of the frame 1 and is disposed across the passage of the needle thread NT extended between the thread tension member and the needle hole, namely perpendicularly across the needle thread NT. The hook 22a of the thread guard 22 hooks the needle thread NT extended between the thread tension member 5 and the thread hole 4a of the thread takeup 4, namely, between the thread guide 6 and the middle thread guide 8. 
     The stopper 24 restricts the length of movement of the hook 22a of the thread guard 22 and is attached under the horizontal surface portion 12d of the support plate 12 to be variable in its position. 
     The stopper 24 is secured to the horizontal surface portion 12d by a screw 26 inserted a long slit 12g provided in the horizontal surface portion 12d while a projecting wall 24a of the stopper 24 is opposed to one end of the operation arm 23 in the swinging direction of the operation arm 23 spaced in a predetermined distance. When the operation arm 23 is swung, one end thereof is brought into contact with the projecting wall 24a whereby the rate or the length of movement of the operation arm 23a is restricted. The rate of movement can be adjusted by sliding the securing position of the stopper 24 along the long slit 12g. 
     An operation of the slackened needle thread assuring unit will be described with reference to FIG. 3. 
     After completion of sewing operation for one cloth, the needle bar 2 is stopped at the lower position for a while on the basis of an electric signal issued by a fixed position stopping unit (not shown) by the operation of a pedal etc., then at the same time when the needle bar 2 starts to move to the upper stop position by the operation of the thread and cutting control unit (not shown), the needle thread NT and the bobbin thread are automatically cut off by the knife of the needle/bobbin thread cutter 7 at the position under the sewn cloth on the basis of an electrical signal issued by a controller (not shown). Thereafter, an end of the needle thread NT thus cut off is held by the thread end holder 10 on the basis of the electric signal issued by the controller. 
     Just after the end of the needle thread NT is held by the thread end holder 10, the solenoid 13 is energized on the basis of the electric signal issued by the controller causing the cylinder rod 14 to be pulled against the resilient force of the spring 15 and to be positioned in the operative position. With a series of operations, as shown in FIG. 3, the bell crank 17 is turned about the pin 19 allowing the operation arm 23 and the thread guard 22 to turn clockwise about the hinge screw 25 via the connecting link 21. As a result, the hook 22a of the thread guard 22 moves forward perpendicularly to the needle thread NT. At this point, the operation arm 23 is brought into contact with the projecting wall 24a of the stopper 24 at one end thereof so that the movement of the operation arm 23 is restricted. With the horizontal movement of the hook 22a in the predetermined length, the needle thread NT having the thread end held by the thread end holder 10 is fed from the neeedle thread supply side, namely, from the spool holder base. Thereafter, when the solenoid is turned off, the operation arm 23 is turned by the resilient force of the spring 15 so that the thread guide 22 is moved backward. As a result, the feeding operation of the needle thread NT is completed thereby assuring sufficient slack of the needle thread NT for forming a normal first stitch. With the movement of the operation arm 23, the cylinder rod 14 is projected upward to return to the inoperative position. 
     When the securing position of the stopper 24 is moved along the long slit 12g by operation of the screw 26, the rate or angle of the turning of the operation lever 23 bringing into contact with the projecting wall portion 24a will be increased or decreased whereby the horizontal movement of the hook 22a of the thread guide 22 is changed so that the feeding length of the upper thread NT is changed. As a result, the amount of slack appropriate for forming the first stitch is assured. 
     Accordingl to the present embodiment, the hook 22a of the thread guide 22 hooks the upper thread NT extended between the thread takeup 4 and the thread tension member 5, but may hook the needle thread NT extended between the thread takeup 4 and the needle 3 to permit the needle thread NT to be fed from the spool holder base. Furthermore, the hook 22a of the thread guide 22 can be constituted to be horizontally reciprocally moved directly by the solenoid as the drive means. 
     With the arrangement of the upper thread and supply assuring unit according to the present invention, inasmuch as the needle thread necessary for forming the first stitch is appropriately assured, there are certain advantages. In the first place the bobbin thread is prevented from drawing over the surface of the sewn cloth because of shortage of slack of the needle thread. Secondly the desired stitch made by the needle thread and the bobbin thread which are well balanced can be formed from the first stitch. Thirdly the working efficiency can be remarkably improved by eliminating the manual mending operation which can take place after completion of the sewing operation and the inconvenience of the cut caused by the shortage of the slack of the needle thread. 
     Although the invention has been described in its preferred form with a certain degree of particularity, it is to be understood that many variations and changes are possible in the invention without departing from the scope thereof.